The present invention relates generally to polarizers and, in particular, to a polarizer employing an optical device which is formed with a combination of birefringent prisms and isotropic mediums to improve transmittance of light rays. The invention can be used in connection with liquid crystal displays, laser optical devices and the like.
There are three types of conventional linear polarizers that are commonly used: (a) absorption type, (b) reflection type, and (c) refraction type. The absorption type polarizer, (e.g., a dichroic sheet polarizer) absorbs one component of an unpolarized light and transmits the component normal thereto. The reflection type (e.g., a polarizing beam splitter) reflects one component of an unpolarized light and transmits the component normal thereto. The refraction type (e.g., a light displacing prism) refracts one component of unpolarized light and transmits the component normal thereto. In these polarizers, at least half of the unpolarized light energy is absorbed, reflected or refracted. Accordingly, the remaining intensity transmitted through the polarizer is less than 50% of the incident light intensity originally received by the polarizer.
In order to improve the transmittance of a conventional polarizer, the inventors of the present application suggest, in JP Application No. 327608/89, the application of a sheet preliminary polarizer as shown in FIG. 1 of the drawings. This preliminary polarizer is formed with two sections: a light separating section 100 and a polarization rotating section 101. Unpolarized incident light 102 is first transmitted to light separating section 100. Unpolarized incident light 102 is formed with two components: a perpendicular polarizing direction 103 with respect to the paper (the first component) and a horizontal polarizing direction 104 with respect to the paper (the second component).
Unpolarized incident light 102 is transmitted to light separating section 100 through a first crystal 109 having an optical axis 109' then to a second crystal 110 having an optical axis 110'. Unpolarized incident light 102 is divided into two beams at the interface of the two edge prisms 109 and 110. Unpolarized incident light 102 then travels as two beams through polarizing rotating section 101. A first beam 105 formed by first component 103 is transmitted light having a polarizing direction rotated 90.degree. in polarizing rotating section 101 and has a horizontal polarizing direction 107 with respect to the paper. A second beam 106 formed by second component 104 is transmitted having a polarization direction 108 parallel to the paper.
In this embodiment, the sheet preliminary polarizer has high transparency and perfectly polarizes unpolarized light. In the case of a normally incident monochromatic light, more than 50% of the incident light is transmitted through the sheet of a preliminary polarizer which is better than a conventional polarizer.
However, the preliminary polarizer disclosed in JP Application No. 327608/89 also has problems. The incident light transmitted through the sheet preliminary polarizer is divided into two light rays having a very small separation angle. Therefore, when an observer watches an object through the sheet preliminary polarizer, two overlapped images are viewed.
It is therefore desirable to provide a polarizer having a combination of birefringent prisms and isotropic mediums to form a section for adjusting two light rays in the same direction. The light ray combining section should have high transmittance so that the transmittance after passing through the polarizer is much greater than a conventional polarizer.